The present invention relates to the enantiomeric enrichment and stereoselective synthesis of chiral amines.
The biological activity of chemical products such as pharmaceuticals and agricultural products which possess a center of chirality often is found to reside principally in one of the possible chiral forms. Since most chemical syntheses are not inherently stereoselective, this poses a serious chemical processing problem. Enrichment in favor of one chiral form thus will be required at some stage, either the final chiral compounds or chemical precursors which possess the same center of chirality. Whatever stage is selected for the enrichment, and in the absence of a method of recycling of the unwanted enantiomer, the process is inherently limited to a maximum theoretical yield of 50% for the desired enantiomer.
Many of the chiral compounds of this type are amines. Moreover because of their synthetic versatility, amines also are good candidates for resolution, after which stereoselective conversion to the chiral compound can be effected. Chemical production of a chiral amine free of its enantiomer heretofore has relied largely on resolution of a mixture of the two chiral forms through formation of diastereomeric derivatives such as a salt with a chiral acid, stereoselective syntheses, or the use of chiral chromatographic columns. See for examples U.S. Pat. No. 3,944,608 and EP-A 36,265.
Some structural types of amines lend themselves to enzymatic resolution. Enzymatic reactions involving .alpha.-amino acids are well known and their use has been proposed for stereospecific preparations. U.S. Pat. No. 3,871,958, for example, discloses the enzymatic preparation of derivatives of the .alpha.-amino acid serine by coupling an aldehyde with glycine in the presence of a threoninealdolase, derived from an E. coli species, as well as a related synthesis of serinol employing ethanolamine.
Relatively little has been reported on enzymatic reactions on amino acids in which the amino group is not vicinal to a carboxylic acid group. Yonaha et al., Agric. Biol. Chem., 42 (12), 2363-2367 (1978) describe an omega-amino acid:pyruvate transaminase found in a Pseudomonas species for which pyruvate was the exclusive amino acceptor. This enzyme, which had been previously crystallized and characterized {see Yonaha et al., Agric. Biol. Chem., 41 (9), 1701-1706 (1977)} had low substrate specificity for omega amino acids such as hypotaurine, 3-aminopropane sulfonate, .beta.-alanine, 4-aminobutyrate, and 8-aminooctanoate and catalyzed transaminations between primary aminoalkanes and pyruvate.
Nakano et al., J. Biochem., 81, 1375-1381 (1977) identified two omega-amino acid transaminases in B. cereus: a .beta.-alanine transaminase, which corresponds to Yonaha et al.'s omega-amino acid:pyruvate transaminase, and a -aminobutyrate transaminase. The two could be distinguished by their dramatically different activities on .beta.-alanine (100 vs. 3) and -aminobutyrate (43 vs. 100), respectively, as well as their distinct amino acceptor requirements.
Burnett et al., J.C.S. Chem. Comm., 1979, 826-828, suggested omega-amino acid:pyruvate transaminase and -aminobutyrate transaminase exhibit different preferences for the two terminal hydrogen atoms in tritium labelled -aminobutyrate.
Tanizawa et al., Biochem. 21, 1104-1108 (1982) examined bacterial L-lysine-.epsilon.-aminotransferase and L-ornithine-.delta.-aminotransferase and noted that while both are specific for L-amino acids, they act distally and with the same stereospecificity as the -aminobutyrate transaminase studied by Burnett et al., supra.
Yonaha et al., Agric. Biol. Chem,, 47 (10), 2257-2265 (1983) subsequently characterized omega-amino acid:pyruvate transaminase and -aminobutyrate transaminase (EC 2.6.1.18 and EC 2.6.1.19) and documented their distribution in a variety of organisms.
Waters et al., FEMS Micro. Lett., 34 (1986) 279-282, reporting on the complete catabolism of .beta.-alanine and .beta.-aminoisobutyrate by P. aeruginosa, noted that the first step involved transamination with .beta.-alanine:pyruvate aminotransferase.
Enzymatic methods have been considered as a method for separating mixtures of chiral amines which are not amino acids, as for example 2-aminobutanol. Most of these involve derivatization, particularly of the amino group, and utilization of this protected group or another group in the molecule to effect separation. EP-A 222,561, for example, describes a process in which racemic 2-aminobutanol is converted to an N-carbamoyl derivative which then is brought into contact with an alkyl alkanoate in the presence of a lipase enzyme. Esterification of the free hydroxy group apparently is limited to the S-enantiomer of the N-carbamoyl derivative, which is thereafter hydrolysed. This process of course is necessarily limited to amines carrying an esterifiable hydroxy group and, moreover, specifically requires prior protection of the amino group through formation of --NH--CO-- carbamoyl group in order to obtain stereospecificity in enzymatic reaction.
EP-A 239,122 describes a similar process applicable to the broader class of 2-amino-1-alkanols.
Japanese Kokai JP 55-138,389 describes the preparation of vicinal amino alcohols by subjecting an alkyl or aralkyl substituted ethyleneimine to microorganisms of the genus Bacillus, Proteus, Erwinia, or Klebsiella.
Japanese Kokai JP 58-198,296 discloses a process in which d,l N-acyl-2-aminobutanol is subjected to the action of an aminoacylase derived from various species of Asperigillus, Penicillium, and Streptomyces which hydrolyses only the d-N-acyl-2-aminobutanol.
Japanese Kokai JP 59-39,294 describes a process for resolving racemic 2-aminobutanol through formation of an N-acetyl derivative which is treated with a Micrococcus acylase to give 1-2-aminobutanol and d-N-acetyl-2-aminobutanol, the latter then being chemically hydrolysed to afford d-2-aminobutanol.
Japanese Kokai JP 63-237796 describes a process in which R,S-1-methyl-3-phenylpropylamine is cultured aerobically in a variety of specified microorganisms with the S-form being metabolized preferentially. The highest yields and optical purity is reported for the yeast species Candida humicola and Trichosporon melibiosaceum. The enzymatic nature of the metabolism of the S-form which occurs in these aerobic cultures, e.g., an oxidase, dehydrogenase, ammonia lysase, etc., is not indicated.
The abstract of Japanese Kokai JP 63-273486 discloses the microbial synthesis of 1-(4-methoxyphenyl)-2-aminopropane with the R-configuration at one of the two chiral centers from 1-(4-methoxyphenyl)-2-propanone with Sarcina lutea.